1. Field of the Invention
This invention pertains generally to treatment of a patient by stereotactic radiosurgery, and more particularly to optimizing dose delivery through inverse treatment planning.
2. Description of the Background Art
Stereotactic radiosurgery is a well-known treatment option available to an oncologist. The procedure can be carried out using a Leksell gamma unit (LGU) commonly referred to as a "Gamma Knife" (marketed by Elekta Corporation, Stockholm, Sweden), a linear accelerator, or similar apparatus. In treating a patient with such an apparatus, the goal of the oncologist is to deliver one or more "doses" so as to maximize irradiation of a target volume of diseased tissue while minimizing irradiation of surrounding healthy tissue. This requires careful and well defined planning prior to treating the patient.
Currently, radiosurgical treatment planning is performed in an iterative manner where the planner uses experience to "guess" at a plan to treat a given target volume. The planner chooses how many "shots" or isocenters to use, as well as the location in three dimensions, the collimator size, and the weighting to be used for each isocenter. A treatment planning computer calculates the dose distribution resulting from this guess, and the efficacy of the plan is evaluated by comparing the resulting isodose lines with the target volume as delineated by contour lines drawn on a series of planar CT (computed tomography) or MRI (magnetic resonance imaging) slices. The planner attempts to improve the plan by moving isocenters, changing weights, and so forth, so that the calculated isodose lines give a better match to the target contours. Eventually, after several iterations, the plan is judged to match the target, or is accepted as the best practical match in consideration of the time constraints imposed on the number of iterations that can be tried.
Various approaches to computerized treatment planning have been developed to assist the planner in his or her optimization of dose patterns. However, when the target volume is large and has a complicated shape, it is very difficult for a human planner to produce a good plan in a reasonable time, even with the assistance of a computer.
The present invention is directed to "inverse" treatment planning. Prior to the present invention, however, inverse treatment planning for radiosurgery performed with an apparatus such as a Leksell Gamma Knife has not been practical. For example, methods which use an unconstrained optimization tend to use many shots of the smallest aperture size. Such an approach is not practical, especially with large volumes, because the time required to treat a given target volume with a very large number of shots is prohibitive. We have discovered that the number of shots generally should not exceed fifteen, and that using a smaller number of shots with larger aperture sizes can often produce almost as satisfactory a plan. We have further discovered that, if optimization based on isodose line matching of the target contour is initiated without first generating an approximate estimate of number of shots and location, then long computation times will be involved.
Therefore, there is a need for an apparatus and method for optimizing radiosurgical dose delivery and shape which is accurate and efficient. The present invention satisfies those, as well as other needs, and overcomes the deficiencies in prior methods and devices.